Software Design Lecture Notes
Fall 2004

For Friday (which is Monday)

1) Read Chapter 14

2) Do Warmup 6


Leftovers
---------

Do the "try this..." examples from notes13.txt

1) list comprehensions

2) DSU pattern


Classes
-------

Big dose of new vocabulary:

class, instance, instantiate, object, constructor, attribute

LEARN IT, SPEAK IT, MAKE IT A PART OF YOU.


A class is a user-defined type.

class Point:
    pass             # later there will be method definitions here

class Rectangle:
    pass


Creating a class defines a function with the same name:

blank = Point()
schmectangle = Rectangle()

These functions are called constructors.

Invoking a constuctor creates an instance of the class.

An instance is a member of a class.

An object is something that can be referred to.

A reference to an object can be assigned to a variable,
passed as an argument or returned as a return value.

In Python:

All instances are objects (can be referred to...)

AND

all objects are instances (belong to some class).

In casual speech, object and instance are used interchangeably.



Attributes
----------

Objects have attributes.

An attribute is like a variable that belongs to an object.

(recall that a variable is a name that refers to a value)

Try this:

from World import *

world = TurtleWorld()
bob = Turtle(world)
print bob.x, bob.y
print bob.color

bob.color = 'blue'
print bob.color

What color is bob?


You can add a new attribute to an object at any time!

bob.new_feature = 'whatever you want'
print bob.new_feature

Does this mean that objects with the same type don't
always have the same attributes?


UML State diagrams with objects
-------------------------------

Usual way to draw objects is a rectangle with

1) the type of object outside

2) the attributes and their values inside


Examples from World.py

class World(Gui):

    def __init__(self):
        self.animals = []
	# other stuff omitted
        
    def register(self, animal):
        self.animals.append(animal)

class Turtle(Animal):

    def __init__(self, world):
	self.world = world
        world.register(self)
	# other stuff omitted


Draw a state diagram showing a World with three Turtles.

Draw a state diagram showing your data structure from Homework 5.

In a stack diagram, the objects can be inside or outside frames.



Secret implementation detail
----------------------------

All user-defined objects are really, secretly, internally...

    DICTIONARIES!!!

Try this:

class Point: pass

blank = Point()
blank.x = 100
blank.y = 200
blank.pointless = 'hello'

print blank.__dict__
{'y': 200, 'x': 100, 'pointless': 'hello'}


A more detailed UML diagram of a user-defined object
would show __dict__, but let's not do that.